Well into "One Nation Under Dog," HBO's revealing but difficult documentary about our deep bonds with canine companions, we reach a point that is nearly too awful to watch.

With plenty of warning to viewers, the film shows what happens at an animal shelter's last stop — a fate met by a couple of million dogs each year in the United States. A half-dozen dogs are set into a large metal box. They seem eerily resigned to this moment; no snapping, no squirming, no escape attempts. The lid is shut and sealed, and a worker twists the gas valve. The screams from inside the box are ghastly.

Next they put in the puppies.

I'm guessing about 80 percent of you aren't reading anymore. Which probably makes "One Nation Under Dog," airing tonight as part of HBO's impressive lineup of summertime documentaries, one of the toughest sells around. This film is only for the most rational sort of dog lover — someone who "gets" the human-dog connection at its most soulful level but also harbors no magical thinking about a dog's life. They are not like us, and we outlive them. Between those two facts exists something almost indescribably dear and fragile.

I admire how the film, which is split thematically into stories of "fear," "loss" and "betrayal," zeroes in unflinchingly on the most troubling aspects of American pet ownership, without becoming an animal-rights brochure. Even though a viewer keeps expecting a Sarah McLachlan ballad to kick in during all those glimpses of sad eyes peering from behind chain-link kennels, the film ends on a sober, imploring note of urgency about spaying and neutering.

"One Nation Under Dog" is inspired by a 2009 book of the same name that was written by Washington City Paper editor Michael Schaffer, who crisscrossed the nation to report on the ways that our pets had left the realm of lesser creatures and become a highly courted niche market. American consumers spend more than $43 billion each year on pet care and pampering. Now, as Schaffer explored, our furred friends are inching their way toward a legal status that treats them not quite as animals and not quite as persons — not yet.

Book not as gloomy

The book was much broader on the subjects of irrational love, playful joy, silly spoiling and lasting companionship, with a denouement on grief and loss. The film, however, is a much darker journey into American doghood, opening with the uneasy tale of a suburban New Jersey doctor, Robert Taffet, whose five Rhodesian Ridgebacks were accused of terrorizing their upscale neighborhood as they ran unleashed and bit at least two people. One ripped a toddler's ear off.

The camera follows the township hearings, court dates, reprimands, warnings. Taffet is a study in a particular kind of dog owner, calmly and defiantly equating his rights and his dogs' rights with those whose teeth-marked injuries would demonstrate that something in that sense of entitlement has gone astray. Eventually, we accompany Taffet and one of the dogs to the vet's office, where the pet is euthanized.

"One Nation Under Dog" assigned different film crews in different directions to document aspects of an overall story, and the result can seem somewhat scattershot and inconclusive. The film is trying to say something all-encompassing about dogs, but what? In a grief support group for people whose dogs have died, we encounter a similar grasping for the right words as the bereaved try to explain not only the love they experienced, but also the lack of sympathy they've received from friends. Losing a dog, they insist, hurts as much or more as losing a person — a sentiment that other dog owners may agree with and yet keep closeted about.

From there, "One Nation Under Dog" is mainly a guided tour: Here is the couple who spent $155,000 having their yellow Lab cloned into a replacement puppy. Here is the high-end dog cemetery and silk-lined dog casket for one pooch's eternal rest.

Ends on a good note

"One Nation Under Dog" finds its strongest narrative and reassuring uplift, following those dedicated souls who rescue shelter dogs and find new homes for them.

One man from New England, John Gagnon, travels to a Tennessee shelter, where, faced with an array of needy cases in the barking cacophony, he makes the difficult choice to save two dogs on death row. One is an angry biter; the other is a placid and terribly sad pit bull spotted with mange.

After some purposeful foster care, the biter winds up living a grand life as the new pet at a beach house; the other finds herself riding in a Fourth of July parade float with her new masters. The gratitude these dogs feel is plain to see. The viewer is grateful to Gagnon, too, for saving "One Nation Under Dog" from utter despair.